Typically aftermarket buttstocks will use a QD sling mounting system. This specific Magpul Industries Sling Mount Kit is a way to add a QD spot for Magpul UBR and STR stocks. Most likely all you would need is the QD button like the one found in the link below. If you have anymore questions or concerns, feel free to email us at Techsales@opticsplanet.com.
https://www.opticsplanet.com/magpul-qd-sling-swivel.html
Each type comes with one mount. To run ambidextrous you will need one of each.
While both the Gen 1 UBR® and STR® come with TYPE 1 push-button QD swivel sockets pre-installed on the left side of the stock, this Sling Mount Kit – TYPE 2 can be installed on the opposite side to add additional push-button QD Sling Swivels or our own Magpul® QDM.
So it is up to you which kit you want to purchase, based on if you want to install on the standard left side or ambidextrous.
Type 1 is the initial mount and hardware you will need. Type 2 attaches to Type 1 if you want ambidextrous mounting capability.
https://www.opticsplanet.com/magpul-industries-sga-stk-sling-mount-kit-type1.html
You need Type 1 to achieve the QD capability. If you want ambidextrous capability, then you will also need a Type 2 mount.
You would need the hardware that is included with the Type 1 kit for installation. The Type 1 kit can be found here:
https://www.opticsplanet.com/magpul-industries-sga-stk-sling-mount-kit-type1.html
According to Magpul, that stock should have come with the reversible Sling Mount Kit – TYPE 3 adapter, so you can choose which side it's on. However if you want to add one to have one on both sides, then this should work with it to add that second mounting point.
Unfortunately Magpul has not provided any installation instructions on their site.
You would need Type 1, however if you want ambidextrous mounts, then you would need Type 2 in conjunction with Type 1, one for each side.
You would need type 1, however if you want it to be ambidextrous, you would need to pair it with a type 2 on the other side.